Injection pumps for a two-stroke engine in a work apparatus utilize the crankcase pressure as a drive. With the downward movement of the piston in the direction of bottom dead center, an overpressure is built up in the crankcase while, for a subsequent upward movement of the piston, the crankcase pressure drops to an underpressure. The crankcase pressure increases with increasing rotational speed up to a positive maximum which can then remain constant up to the highest engine speed. The pressure fluctuations in the crankcase lie approximately between 0.75 bar and -0.2 bar.
With increasing engine speeds, the air charge in the combustion chamber becomes less because of the increasing flow resistance in the air channels; whereas, the quantity of injected fuel remains unchanged because of the essentially unchanged crankcase pulse. For avoiding an overenrichment of the mixture at high engine speeds, U.S. Pat. No. 4,846,119 suggests connecting the rearward chamber to the atmosphere via a throttle flowpath in order to adapt the injected quantity of fuel. Only at high engine speeds and therefore at rapid movements of the membrane, does this flowpath become effective and effects a counterpressure in the rearward chamber which is built up with increasing engine speed whereby the stroke of the pump piston is reduced and a reduced quantity of fuel is obtained.
In addition to considering this dynamic performance, a static adaptation of the stroke of the injection pump must be ensured and therefore also the pumped quantity of fuel to the position of the throttle flap, that is, to the supplied air quantity. The leaf spring suggested in U.S. Pat. No. 4,846,119 ensures the required adaptation only in an inadequate manner. U.S. Pat. No. 4,846,119 is incorporated herein by reference.